This invention relates generally to reference voltage generators and more particularly, it relates to a bias generator for use in CML circuits which has an output reference voltage that is substantially independent of variations in supply voltage over a wide temperature range.
As is generally known, conventional emitter-coupled logic (ECL) gate circuits can be constructed of a differential transistor circuit with emitter follower output transistors as shown in FIG. 1(a). When the emitter follower output transistors are eliminated, as is shown in FIG. 1(b), this type of logic gate is sometimes referred to as a current mode logic (CML) gate. Such ECL and CML gate circuits are well suited for high performance products such as electronic computers and other electronic apparatus since the differential transistors are operated out of saturation with relatively small voltage swings. As a result, these circuits have small propagation delays, thereby providing a high speed of operation. Since the emitter follower transistors T3 and T4 are not present in the CML gate circuit of FIG. 1(b), CML gate circuits have generally a smaller power dissipation and a smaller voltage swing than the ECL gate circuits. The conventional ECL gate circuit has a logic swing of approximately 650 mV to 850 mV. However, for the CML gate circuits the logic swing is about 340 mV to 430 mV. In view of the fact that both of these types of circuits have a wide range of applications, it has thus become important to preserve their high performance potential when such circuits are designed and fabricated as integrated circuits.
In order to insure that integrated circuits embodying the ECL gate circuits of FIG. 1(a) achieve maximum performance, a conventional bandgap reference voltage V.sub.CS is commonly generated on the same chip and is used to control the base of the main current source transistor T5 which determines the magnitude of the current flowing through the reference transistor T2 or the input transistor T1. The bandgap reference voltage V.sub.CS has the characterisitc of being stable and of tracking variations in processing and temperature as well as changes in operating parameters such as temperature. A conventional bandgap circuit has a typical swing which is maintained within 50 mV over the operating temperature, power supply and process variations. However, such conventional bandgap reference voltage generators have the disadvantage of being susceptible to oscillations. While this oscillation problem can be overcomed with the use of a capacitor, such a requirement is generally an undesirable addition to an integrated circuit since it entails the use of a relatively large amount of chip area. A prior art bandgap generator as just described is illustrated in FIG. 2. As can be seen this bandgap generator circuit suffers further from the disadvantage of requiring the use of a relatively high number of circuit components, thereby increasing power consumption and manufacturing costs.
While the modified prior art bandgap reference circuit of FIG. 2 could be used to adequately generate the reference voltage V.sub.CS of FIG. 2(b), it would be more expedient to provide a bias generator for the CML gate circuit which avoids the possibility of oscillations, eliminates the needs of the feedback capacitor, and contains a reduced number of circuit elements for constructing the circuit. To the best of the inventor's knowledge, no one has heretofore developed a bias generator for CML gate circuits which is of a relatively simple construction as in the present invention.